This invention relates to an acceleration sensor for detecting the action of acceleration and/or deceleration (hereinafter collectively referred to as "acceleration") in a vehicle. For example, such an acceleration sensor is usable for controlling a passive seat belt system in which a webbing is secured at one end thereof on a retractor mounted on a floor and is movable back and forth at the other end thereof along a roof rail in response to the closure and opening of an associated door. The acceleration sensor is also usable for the control of a fuel pump or the like. When the acceleration sensor detects an acceleration, for example, in the event of a vehicular collision, it serves to prevent the other end of the webbing from moving forward along the roof rail even when the door opens and/or to stop the operation of the fuel pump.
Conventional acceleration sensors includes that disclosed in Japanese Patent Publication No. SHO 51-20704. This conventional acceleration sensor is equipped with a spherical inertia element, which is made of a magnetic material and having a predetermined mass, and a plate-like magnet for attracting the inertia element. The inertia element is retained by magnetic force at a first end on an upper wall of the magnet but, when a predetermined acceleration is applied, the inertia element is caused to move to a second end on the upper wall of the magnet so that a switch is turned on. The inertia element is allowed to remain at the second end by magnetic force.
The conventional acceleration sensor described above is, however, accompanied by the problem that, if the inertia element is caused to move to another position by a secondary collision or the like, the switch is turned off and a signal indicating a collision can no longer be continuously outputted.